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Homework Help: Tension on cable

  1. Dec 18, 2009 #1
    1. The problem statement, all variables and given/known data

    A 500 kg beam is free to pivot about its
    lower end and is supported by a horizontal
    cable at its upper end, as shown in the
    figure. (figure has angle 35 degrees between beam and the wall)
    Assuming the mass of the beam is
    uniformly distributed, what is the tension in
    the cable?

    2. Relevant equations

    Torque = r * F

    3. The attempt at a solution

    Using pivot point at the end of the beam, the torque is 0.

    weight acting down on the beam is mg*l/2*sin(35). L is not givin not sure where to go from there.
     
  2. jcsd
  3. Dec 18, 2009 #2

    PhanthomJay

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    What's the torque from the cord tension about the pivot?
     
  4. Dec 18, 2009 #3
    you mean from the top of the beam to the bottom? :uhh:
     
  5. Dec 18, 2009 #4

    PhanthomJay

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    yes.
     
  6. Dec 18, 2009 #5
    Not sure how to find it.
     
  7. Dec 18, 2009 #6

    PhanthomJay

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    You correctly identified the torque about the pivot from the weight force as mgL/2sintheta, that is Torque = r X F = rFsintheta, where F in this case is mg, and r is L/2, and theta is 35 degrees(the angle between the force and position vector). Now do the same for the torque about the pivot for the unknown horizontal tension force (call it T) at the top of the beam, that is, Torque from T = r X F = rFsintheta, where r is L, T is itself, and theta , the angle between T and L , is ????.. Solve for T by summing these 2 torques and setting that sum equal to 0. The 'L" term should cancel out.
     
  8. Dec 18, 2009 #7
    mg*L/2*sin(35) = T*L*sin(55)

    mg/2*sin(35) = T*sin(55)

    (mg/2*sin(35))/sin(55) = T

    T = 1.72 kN

    Thanks :)
     
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